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                    <h1 class="description center-align post-title">从内存结构的角度带你看 String Table</h1>
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                <h2 id="一、String的基本特征"><a href="#一、String的基本特征" class="headerlink" title="一、String的基本特征"></a>一、String的基本特征</h2><h3 id="一、String的基本特征-1"><a href="#一、String的基本特征-1" class="headerlink" title="一、String的基本特征"></a>一、String的基本特征</h3><ul>
<li>String：字符串，使用一对 “ “ 引起来表示。</li>
<li>String 声明为 final 的，不可被继承。</li>
<li>String 实现了 Serializable：表示字符串是支持序列化的；实现了Comparable接口：表示String可以比较大小。</li>
<li>String在 jdk8 及之前内部定义了final char[] value用于存储字符串数据，jdk9时改为了byte[]。</li>
</ul>
<h3 id="二、String在-jdk9-中存储结构变更"><a href="#二、String在-jdk9-中存储结构变更" class="headerlink" title="二、String在 jdk9 中存储结构变更"></a>二、String在 jdk9 中存储结构变更</h3><p>tring再也不用char[]来存储了，改成了byte[] 加上编码标记，节约了一些空间。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115145318.png" alt="image-20220327115145318" style="zoom:80%;" />

<p>那StringBuffer 和 StringBuider是否仍无动于衷呢？StringBuffer 和 StringBuilder 也更新成相同的表示了。</p>
<h3 id="三、String的基本特征"><a href="#三、String的基本特征" class="headerlink" title="三、String的基本特征"></a>三、String的基本特征</h3><p>String：代表不可变的字符序列，简称：不可变性。</p>
<ul>
<li>当字符串重新赋值时，需要重写指定内存区域赋值，不能使用原有的value进行赋值。</li>
<li>当对现有的字符串进行连续操作时，也需要重新指定内存区域赋值，不能使用原有的value进行赋值。</li>
<li>当调用String的 replace() 方法修改指定字符或字符串时，也需要重新指定内存区域赋值，不能使用原有的value进行赋值。</li>
</ul>
<p>通过字面量的方式（区别于new）给一个字符串赋值，此时的字符串值声明在字符串常量池中，比如下面这样：</p>
<pre class="line-numbers language-none"><code class="language-none">String a &#x3D; &quot;aaa&quot;;<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<blockquote>
<p>字符串常量池中是不会存储相同内容的字符串的</p>
</blockquote>
<p>String 的 String Pool 是一个固定大小的 Hashtable，默认值长度是 1009。如果放进去String Pool 的String 非常多，就会造成 Hash 冲突严重，从而导致链表会很长，而链表长了后会直接造成的影响就是当调用 Sting.intern() 时性能下降。</p>
<p>使用-XX:StringTableSize可以设置 StringTable 的长度，在jdk6中 StringTable 是固定的，就是 1009 的长度，所以如果常量池中的字符串过多就会导致效率下降很快，StringTableSize设置没有要求；在jdk7中，StringTable的默认长度是 60013，StringTableSize 设置没有要求，jdk8开始，设置 StringTable 的长度的话，1009 是可以设置的最小值。</p>
<h2 id="二、String-的内存分配"><a href="#二、String-的内存分配" class="headerlink" title="二、String 的内存分配"></a>二、String 的内存分配</h2><h3 id="一、String-的内存分配"><a href="#一、String-的内存分配" class="headerlink" title="一、String 的内存分配"></a>一、String 的内存分配</h3><p>在 Java 语言中有8种基本数据类型和一种比较特殊的类型 String，这些类型为了使它们在运行过程中速度更快、更节省内存，都提供了一种常量池的概念。</p>
<p>常量池就类似一个Java 系统级别提供的缓存，8种基本数据类型的常量池都是系统协调的，String 类型的常量池比较特殊，它的主要使用方法有两种：</p>
<ul>
<li>直接使用双引号声明出来的String对象会直接存储在常量池中，比如：String info = “ktf is a shuaige”。</li>
<li>如果不是用双引号声明的String对象，可以使用 String 提供的 intern() 方法。</li>
</ul>
<p>Java 6 及以前，字符串常量池存放在永久代，Java 7中 Oracle 的工程师对字符串的逻辑做了很大的改变，即将字符串常量池的位置调整到 Java 堆内。所有的字符串都保存在堆（Heap）中，和其他普通对象一样，这样就可以让你在进行调优应用时仅需要调整堆大小就可以了。</p>
<p>字符串常量池概念原本使用的比较多，但是这个改动使得我们有足够的理由让我们重新考虑在 Java7 中使用String.intern()。Java 8方法区改成了元空间实现，但是字符串常量池还是在堆。</p>
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<h2 id="三、Sting的基本操作"><a href="#三、Sting的基本操作" class="headerlink" title="三、Sting的基本操作"></a>三、Sting的基本操作</h2><p>Java 语言规范里要求完全相同的字符串字面量，应该包含同样的 Unicode 字符序列（包含同一份码点序列的常量），并且必须是指向同一个 String 类实例。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">Memory</span> <span class="token punctuation">&#123;</span>
	<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> main <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>    <span class="token comment">// line 1</span>
		<span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>   <span class="token comment">// line 2</span>
		<span class="token class-name">Object</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>     <span class="token comment">// line 3</span>
		<span class="token class-name">Memory</span> mem <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Memory</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>      <span class="token comment">// line 4</span>
		mem<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>     <span class="token comment">// line 5</span>
	<span class="token punctuation">&#125;</span>     <span class="token comment">// line 9</span>
	
	<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token class-name">Object</span> param<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>        <span class="token comment">// line 6</span>
		<span class="token class-name">String</span> str <span class="token operator">=</span> param<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>     <span class="token comment">// line 7</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>       
	<span class="token punctuation">&#125;</span>   <span class="token comment">// line 8</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="四、字符串拼接操作"><a href="#四、字符串拼接操作" class="headerlink" title="四、字符串拼接操作"></a>四、字符串拼接操作</h2><ul>
<li>常量与常量的拼接结果在常量池，原理是编译器优化。</li>
<li>常量池中不会存在相同的常量。</li>
<li>只要其中一个是变量，结果就在堆中，变量拼接的原理是StringBuilder。</li>
<li>如果拼接的结果调用 intern（）方法，则主动将常量池中还没有的字符串对象放入池中，并返回此对象地址。</li>
</ul>
<p><img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115205813.png" alt="image-20220327115205813"></p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115210211.png" alt="image-20220327115210211" style="zoom:80%;" />

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115221938.png" alt="image-20220327115221938" style="zoom:80%;" />

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115230042.png" alt="image-20220327115230042" style="zoom:80%;" />

<h2 id="五、intern-的使用"><a href="#五、intern-的使用" class="headerlink" title="五、intern()的使用"></a>五、intern()的使用</h2><img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115237739.png" alt="image-20220327115237739" style="zoom:80%;" />

<p>如果不是用双引号声明的 String 对象，可以使用 String 提供的 intern 方法：intern 方法会从字符串常量池中查询当前字符串是否存在，若不存在就会将当前字符串放入常量池中。</p>
<p>比如：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">String</span> myInfo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">"I love you"</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<blockquote>
<p>也就是说，如果在任意字符串上调用 String.intern 方法，那么其返回结果所指向的那个类实例，必须和直接以常量池形式出现的字符串实例完全相同，因此下列表达式的值必定是true。</p>
</blockquote>
<pre class="line-numbers language-jva" data-language="jva"><code class="language-jva">(&quot;a&quot; + &quot;b&quot; + &quot;c&quot;).intern() &#x3D;&#x3D; &quot;abc&quot;<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>通俗点讲，Interned String 就是确保字符串在内存里只有一份拷贝，这样可以节约内存空间，加快字符串操作任务的执行速度。注意，这个值会被存放在字符串内部池（String Intern Pool）。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115244648.png" alt="image-20220327115244648" style="zoom:80%;" />

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115251534.png" alt="image-20220327115251534" style="zoom:80%;" />

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115258057.png" alt="image-20220327115258057" style="zoom:67%;" />

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115304459.png" alt="image-20220327115304459" style="zoom:67%;" />

<p>总结String的intern() 的使用：</p>
<ul>
<li>jdk1.6中，将这个字符串对象尝试放入字符串常量池中：<ul>
<li>如果串池中有，则并不会放入，返回已有的串池中的对象的地址。</li>
<li>如果没有，会把此对象复制一份，放入串池，并返回串池中的对象地址。</li>
</ul>
</li>
<li>jdk1.7起，将这个字符串常量池尝试放入串池：<ul>
<li>如果串池中有，则并不会放入，返回已有的串池中的对象的地址。</li>
<li>如果没有，则会把对象的引用地址复制一份，传入串池，并返回串池中的引用地址。</li>
</ul>
</li>
</ul>
<h2 id="六、StringTable-的垃圾回收"><a href="#六、StringTable-的垃圾回收" class="headerlink" title="六、StringTable 的垃圾回收"></a>六、StringTable 的垃圾回收</h2><img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115321439.png" alt="image-20220327115321439" style="zoom:80%;" />

<h2 id="七、G1-中的-String-去重操作"><a href="#七、G1-中的-String-去重操作" class="headerlink" title="七、G1 中的 String 去重操作"></a>七、G1 中的 String 去重操作</h2><p>背景：对许多Java引用（有大的有小的）做的测试得出以下结果：</p>
<ul>
<li>堆存活数据集合里面 String 对象占了25%。</li>
<li>堆存活数据集合里面重复的 String 对象有13.5%。</li>
<li>String 对象的平均长度是 45。</li>
</ul>
<p>许多大规模的 Java 引用的瓶颈在于内存，测试表明，在这些类型的应用里面，Java堆中存活的数据集合差不多25% 是 String 对象，更进一步，这里面差不多一半String对象是重复的，重复的意思是说：String1.equals(String2) = true，堆上存在重复的String对象必然是一种内存的浪费。这个项目将在G1垃圾回收器中实现自动持续对重复的String 对象进行去重，这样就能避免浪费内存。</p>
<p>实现：</p>
<ul>
<li>当垃圾回收器工作的时候，会访问堆上存活的对象，对每一个访问的对象都会检查是否是候选的要去重的String对象。</li>
<li>如果是，把这个对象的一个引用插入到队列中等待后续的处理。一个去重的线程在后台运行，处理这个队列。处理队列的一个元素意味着从队列删除这个元素，然后尝试去重它引用的String对象。</li>
<li>使用一个hashtable来记录所有的被String对象使用的不重复的char数组。当去重的时候，会查这个hashtable，来看堆上是否已经存在一个一模一样的char数组。</li>
<li>如果存在，String对象会被调整引用那个数组，释放对原来的数组的引用，最终会被垃圾收集器回收掉。</li>
<li>如果查找失败，char数组会被插入到hashtable，这样以后的时候就可以共享这个数组了。</li>
</ul>
<p>命令行选项：</p>
<ul>
<li>UseStringDeduplication(bool)：开启String去重，默认是不开启的，需要手动开启。</li>
<li>PrintStringDeduplicationStatistics（bool）：打印详细的去重统计消息。</li>
<li>StringDeduplicationAgeThreshold（uintx):达到这个年龄的String对象被认为是去重的候选对象。</li>
</ul>

                
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            <a href="/about" target="_blank">Gtwff</a>
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